Datasheet

34
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
Notes: 1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These
options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability
at start-up. They can also be used with crystals when not operating close to the maximum fre-
quency of the device, and if frequency stability at start-up is not important for the application.
9.4 Full swing Crystal Oscillator
This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is
useful for driving other clock inputs and in noisy environments. The current consumption is
higher than the ”Low Power Crystal Oscillator” on page 33. Note that the Full Swing Crystal
Oscillator will only operate for Vcc = 2.7 - 5.5 volts.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 9-6. The
crystal should be connected as described in ”Clock source connections” on page 32.
The operating mode is selected by the fuses CKSEL3..1 as shown in Table 9-5.
Notes: 1. If the crystal frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured
that the resulting divided clock meets the frequency specification of the device.
Notes: 1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These
options are not suitable for crystals.
Table 9-5. Full Swing Crystal Oscillator Operating Modes.
Frequency range
(1)
[MHz] CKSEL3..1 Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 20 011 12 - 22
Table 9-6. Start-up times for the Full Swing Crystal Oscillator Clock Selection.
Oscillator source /
power conditions
Start-up time from
power-down and
power-save
Additional delay
from reset
(V
CC
= 5.0V) CKSEL0 SUT1..0
Ceramic resonator,
fast rising power
258CK 14CK + 4.1ms
(1)
000
Ceramic resonator,
slowly rising power
258CK 14CK + 65ms
(1)
001
Ceramic resonator,
BOD enabled
1K CK 14CK
(2)
010
Ceramic resonator,
fast rising power
1K CK 14CK + 4.1ms
(2)
011
Ceramic resonator,
slowly rising power
1K CK 14CK + 65ms
(2)
100
Crystal Oscillator,
BOD enabled
16K CK 14CK 1 01
Crystal Oscillator,
fast rising power
16K CK 14CK + 4.1ms 1 10
Crystal Oscillator,
slowly rising power
16K CK 14CK + 65ms 1 11